This invention relates to vertical deflection circuits and more particularly to switched mode vertical deflection circuits.
Most vertical deflection systems for television receiver use include linear amplifiers for amplifying a sawtooth voltage wave. The output stages in such systems may use single-ended or push-pull circuit configurations for driving a sawtooth of current through the vertical deflection winding. Measurements have shown that the vertical deflection output stage dissipates power in amounts which in some cases may be up to twice or more of the power consumed by the deflection winding.
More efficient vertical deflection circuits have been proposed utilizing class-D operated amplifier output circuits. In a class-D amplifier the output transistors are operated as switches, and since the transistors usually are either nonconducting or saturated when so operated, the power dissipation in the transistors is reduced. To achieve the required vertical rate scanning current waveform, it is common to pulse-width modulate a higher frequency signal, such as the horizontal rate signal or a multiple thereof, at the vertical deflection rate and use these pulse-width modulated signals to drive the class-D output stages. To remove the horizontal rate component from the vertical scanning current sometimes it is necessary to utilize filter networks which consume a relatively large amount of power, thereby offsetting the advantages of a class-D amplifier to some extent.
Another serious consideration in the use of class-D amplifiers is the minimizing of crossover distortion. Crossover distortion occurs when the scanning current sawtooth waveform is not linear when it passes through zero and reverses polarity at the middle of the vertical trace interval. Such distortion resulting from the nonlinear current manifests itself as an increased intensity horizontal bar across the center of the viewing screen. In other situations in which the class-D circuits produce a horizontal rate triangular current component on the vertical scanning current a diagonal line may appear on the viewing screen.